The invention relates to the preparation and use of dispersions or solutions comprising optionally substituted polythiophenes in anhydrous or low-water-content organic solvents.
Conductive polymers based on substituted thiophenes are increasingly being used in industry, e.g., in the through-plating of circuit boards (EP-A 553671), the antistatic finishing of photographic films and plastic moldings (EP-A 440957) or for electrodes in solid-electrolyte capacitors (EP-A 340512). For uses of this type, dispersions of π-conjugated, polymeric compounds of this type are preferably employed.
Aqueous dispersions or solutions of polythiophene+An− ion complexes have proven to be particularly suitable for such purposes since they have both high stability and result in coatings having excellent conductivity.
These polythiophene+An− ion complexes may consist of π-conjugated polythiophenes, whose positive charges are delocalized over the entire molecule, and anions of, for example, organic polyacids, which compensate for these positive charges.
In the oxidative polymerization of substituted 3,4-alkylenedioxythiophenes using potassium peroxodisulfate as an oxidant in the presence of polystyrenesulfonic acid, EP-A 440957 states that polythiophene+An− ion complexes are formed as aqueous dispersions which consist of 3,4-polyalkylenedioxythiophenes and anions of polystyrenesulfonic acid. These dispersions can be employed directly for the above-mentioned purposes.
The oxidative polymerization of substituted 3,4-alkylenedioxythiophenes which are substituted in the alkylene unit by radicals carrying acid groups likewise results in the formation of dispersions or solutions of polythiophene+An− ion complexes, but in which the positive charges of the conjugated π-system are compensated by the acid anions likewise already present in the molecule.
An improved variant for the preparation of aqueous dispersions or solutions of this type is the use of ion exchangers for removing the inorganic salt content, or part thereof, originating predominantly from the chemical oxidation (DE-A 19 627 071). This desalination step prevents the formation of cloudiness and inhomogeneities (for example in the production of thin antistatic layers) and helps form highly transparent, clear layers.
However, it has been found that, in spite of the good properties of aqueous dispersions or solutions, the different wetting ability compared with water and the differentiated drying behavior of organic solvents is advantageous for some applications, e.g., for the application of conductive layers to the support material.
Simple drying or removal of the water from the above-mentioned dispersions or solutions by distillation gives powders which cannot be redispersed in a simple manner by addition of organic solvents.
EP-A 203 438 discloses polymers of substituted thiophenes which are dispersed in organic solvents. However, the disadvantage of the process described therein is the preparation of the polythiophenes from substituted 2,5-dihalothiophenes with the aid of magnesium in the presence of a nickel catalyst. It is not practicable to carry out the reaction in this way on a relatively large scale, and the content of carcinogenic and allergenic nickel prevents the use of the solution without prior, complex work-up steps.
EP-A 253 994 likewise describes a method for the preparation of solutions or dispersions of polythiophene+An− ion complexes consisting of polythiophenes and anions of the conductive salts used therein, in organic solvents. In these cases, the polymerization of the monomeric thiophenes takes place through chemical oxidation in the organic solvent itself. In this process, however, the desired product precipitates out of the reaction solution and therefore cannot be used, for example, for the production of transparent films.
According to EP-A 440 957, dispersions comprising 3,4-polyalkylenedioxythiophenes can be prepared directly in organic solvents, but the inorganic salt content, predominantly originating from the chemical oxidation, remains in solution in such cases, which may lead to the above-mentioned undesired effects in the production of coatings.
There was therefore a need to find a method of combining the excellent conductivity and coating properties of the fully or partially desalinated, aqueous dispersions or solutions of the polythiophene+An− ion complexes with the widely variable wetting and drying properties of organic solvents.